Hi Rebecca.
It depends what you mean by long! If you mean long, as in long time, then the answer would be many hours, but I don't think people keep track of that.
What we do try to keep track of is the flare that produces the most energy. It is a little hard to do because the brightest flares are too bright for our instruments to observe correctly! There was one which was especially bright on November of 2003. A large flare releases something like 10 million times the energy in a big volcanic explosion.
Here is the list of the brightest flares we have observed since we started measuring them from space:
http://www.spaceweather.c...lares/topflares.html
"X" flares are the brightes tkind and the higher the number after the X the brighter the flare.
Here is some stories related to the Nov. 4, 2003 flare:
http://sohowww.nascom.nas...hotshots/2003_11_04/
http://science.nasa.gov/s.../2003/12nov_haywire/
Some stars other than our Sun produce even brighter flares. See this story:
http://www.nasa.gov/cente.../pipsqueak_star.html